Project information

Project financing:

Elitenetzwerk Bayern

Abstract

Polymer-plasticizer systems show a decoupling of the dynamics of both components and the existence of large dynamic heterogeneities especially for the low-Tg (plasticizer) component. The question arises, whether these effects depend on the polymer character or whether they are intrinsic to binary systems, including non-polymeric systems. Therefore, we studied the low-Tg component tricresyl phosphate (TCP, Tg = 206 K) mixed with a specially synthesized high-Tg spirobichroman derivative (DH 379, Tg = 382 K) and a second mixture consisting of tripropyl phosphate (TPP, Tg = 134 K) and another spirobichroman derivative (SBC, Tg = 356 K). The dynamics of TCP and TPP were probed by 31P NMR in the whole concentration range, while the SBC component was deuterated and thus was selectively investigated by 2H NMR. Different NMR methods (Echo spectra, two-dimensional NMR spectra, stimulated echo measurements, analysis of the spin-lattice and spin-spin relaxation times) reveal the reorientational dynamics of the main and secondary relaxation.
For all concentrations, two main relaxation processes with two distinct Tg are identified by dielectric spectroscopy (DS), Tg1 and Tg2. The faster one (α2-process) is attributed to the dynamics of the low-Tg component. It shows increasing dynamic heterogeneities of the reorientational dynamics, described by a broad distribution of G(lnτα2), with decreasing concentration of the low-Tg component, as demonstrated by all NMR observables. This is reflected by an increasing stretching of the stimulated echo correlation function, from a more or less exponential decay at high concentrations to a quasi-logarithmic decay at low concentrations. 2D NMR spectroscopy reveals, that the dynamic heterogeneities are of transient nature, i.e. a dynamical exchange between fast and slow moving molecules within the broad G(lnτα2) occurs. The slower process (α1-process) is associated with the dynamics of the high-Tg component. However, there are indications that a fraction of the low-Tg component also takes part in the slow α1-process. In contrast to the α2-process, the α1-process broadens only weakly when adding the low-Tg component and is comparable to the main relaxation in a neat glass former. Furthermore, the plasticizer effect (α1) and the anti-plasticizer effect (α2), respectively, are observed. In the range Tg2 < T < Tg1, i.e. in the arrested high-Tg matrix (α1-process), the molecules of the low-Tg component involved in the α2-process still show an isotropic liquid-like motion as proven with 1D and 2D NMR spectra. Further, a crossover of the temperature dependence of τα2(T) from a non-Arrhenius to an Arrhenius-like behavior, at low concentrations of the low-Tg component, is observed below Tg1. This change in the temperature dependence also leads to a decrease of Tg2 at low concentrations and thus leads to a maximum in Tg2 (clow-Tg). The dynamical behavior of the α2-process is attributed to intrinsic confinement effects.
Exploiting the sensitivity of 2H and 31P NMR echo experiments, the secondary process (β) was studied in the system TPP/SBC in the entire concentration range. The β-process of neat TPP is rediscovered in the mixture with unchanged time constants. The NMR spectra (for short interpulse delays) in the slow and fast exchange limit are very similar with only slightly changed spectral widths (ca. 1%). Thus, a spatially highly restricted motion with angular displacements below ±10° is performed. Yet, increasing the interpulse delay, TPP shows echo spectra typical of a small angle motion with decreasing intensity in the center of the spectra. At the same temperatures, very similar line shape effects are recognized in the 2H NMR spectra of the SBC component in the mixture, not observed in neat SBC. It appears that the TPP molecules enslave the SBC molecules to perform a similar hindered reorientation with the same time constant as TPP. In addition, the temperature dependence of the spin-lattice relaxation becomes the same for both components at low temperature. Taking into account the different coupling constants, the amplitude of the β-relaxation is estimated to be smaller in the high-Tg component. At lowest weight concentrations (cTPP ≤ 20 %) one finds indications that the β-process starts to disintegrate, i.e. not all molecules take part in the β-relaxation any longer, as expected for cTPP → 0. Altogether, we conclude that the β-process is a cooperative process. Summarizing, all dynamical features reported in polymer additive systems or polymer blends are also observed in the non-polymeric systems.
Furthermore, the dynamics of bulk polypropylene imine (PPI) dendrimers of generation 2-5 were investigated. A secondary process, traces of which are observed in the 2H NMR spin-lattice relaxation time and in the 2H NMR spectra, exhibits features characteristic of a typical β-process. Moreover, 2H NMR identifies amino end group jumps with a broad distribution of correlation times G(lnτ). The characteristics of the main relaxation are essentially independent of generation and the dendrimers exhibit a glass point Tg of about 200 K.